1. Field of the Invention
This invention relates generally to a method and apparatus for determining the temperature of the hot spot in a transformer, and more particularly, to such a method and apparatus employing the transformer current and top-oil temperature to determine the hot-spot temperature.
2. Description of the Prior Art
The economics of utility operations require that all components be operated at their maximum rating consistent with a satisfactory operating life. For transformers, the generation of heat due to I.sup.2 R and eddy current losses and the efficiency of the winding cooling system determines the maximum transformer rating. The generation of heat in a transformer is not homogeneous. Local hot spots are produced by imperfections in materials and the manufacturing process. The effect of these hot spots is to accelerate degradation of the insulating material and cause premature failure of the transformer.
Without real-time monitoring of the transformer hot-spot temperature, the transformer is operated below its theoretical rating to mitigate temperature degradation of the insulating materials. Continuous monitoring of the transformer hot spot allows the transformer to be safely operated much closer to the critical temperature. In effect, this greatly improves the electric utility's return on capital investment by allowing expensive transformers to be operated at a significantly higher capacity with no appreciable increase in capital equipment expense. Accurate temperature monitoring, while the transformer is operating, also provides the basis for estimating the remaining thermal life of the insulation, given a known loading history. Typically, a transformer is designed in such a way that, at rated load, the temperature of any winding is approximately 65.degree. C. above the ambient air temperature. In this design process, one of the transformer's windings typically reaches this temperature value before the other windings do. This particular winding will then be used as the basis of the calculated hot spot per ANSI loading procedures known to those skilled in the art. Accurate hot-spot monitoring could also allow possible future redesign of coil cooling features to provide a more uniform temperature distribution.
Several techniques are known in the art for hot-spot temperature monitoring, including: a thermometer producing an electrical signal representative of the hot-spot temperature; an acoustic-signal generator using a mechanical resonator with the resonator frequency dependent on temperature and a separate detector responsive to the acoustic signal; a device for producing an electromagnetic signal having a frequency functionally related to the hot-spot temperature and a separate frequency detector; and an optical fiber with a plurality of cores where the cross-talk between cores is a function of the hot-spot temperature.
In another well-known technique, the hot-spot temperature monitor is mounted away from the transformer hot spot. The monitor includes a mechanical dial indicator with a liquid-filled capillary sensor, a well into which the sensor is placed, a heater inside the well, a current transformer to heat the heater, and a current balancing auto-transformer to adjust the current in the heater. The helical coil in the dial indicator assembly is unwound (or wound) by the contracting (or expanding) liquid in the filled capillary tube. A shaft connected to the center of the helical coil rotates, thus turning the indicating pointer and producing the hot-spot temperature signal. Control of coiling equipment is derived from adjustable cams on the shaft; these cams trip snap action switches to activate and deactivate the cooling equipment.
All of these prior art techniques exhibit at least one or more disadvantages including expense and complexity of electronics. For some of these techniques, it is necessary to run an electrical conductor from the temperature sensor through the transformer insulation, windings, and oil to the exterior of the transformer. This conductor is exposed to both high electric and magnetic stresses. The mechanical dial indicator technique is susceptible to transformer vibration, loss of liquid in the capillary tube, wide variations, in dial indication, and it lacks a fail-safe mode of operation. In general, the major problem with hot-spot temperature sensing is the hostile environment within the transformer through which the temperature data must pass to the transformer exterior.